1. Field of the Invention
The present invention relates to a process for fabricating highly uniform ceramic electroactive transducers. More particularly, the present invention relates to a process for fabricating hollow ceramic electroactive transducers, which are essentially spherical in shape.
2. Description of the Prior Art
A large number of medical procedures which require catheters are performed in the United States each year. Catheters are, typically, plastic tubes having a diameter of few millimeters. The uses of such catheters include, intravenous drug delivery, therapeutic device delivery, and other types of invasive procedures, such as, guiding balloon angioplasties of the leg, guiding catheters inside the heart to ablate incorrectly functioning cardiac tissue, and guiding catheters within the uterus to inject fluid into the fallopian tubes to test for tubal blockage.
Catheters must be guided to the proper location in the subject. One method of guiding catheters is to employ ultrasonic imaging. In doing so, an ultrasonic transducer is mounted on the catheter and a receiver is placed on the subject. When an ultrasound signal from the catheter mounted transducer strikes the external receiver, the signal is converted into an electronic marker corresponding to the precise location of the catheter in the ultrasound image of the subject.
At present, catheters are guided with real time X-ray imaging known as fluoroscopy. The use of ultrasound imaging has not found wide acceptance despite the advantages ultrasound has over X-ray technology.
A major encumbrance to using ultrasound imaging in guiding catheters is orientation-dependent ultrasound visibility of catheters known in the art. The orientation-dependent visibility, which plagues this technology, results from the wavelength of the ultrasound being several times smaller than the catheter is wide. The result is a catheter that becomes an ultrasound reflector, in addition to being an ultrasound transducer.
This produces an ultrasound visibility that is highly dependent on the orientation of the mounted transducer. X-ray fluoroscopy is not hindered by this phenomenon.
In one approach to eliminate orientation dependant ultrasonic visibility, an omnidirectional ultrasonic transducer is directly mounted on one end of the catheter. This approach solves the problem of signal orientation dependence on catheter orientation. To be suitable for this use, the ultrasonic transducer must be omnidirectional, possess low signal loss, have a high sensitivity, and preferably be uniform in construction and inexpensive to produce. Use of such an omnidirectional ultrasonic transducer mounted on a catheter would thus allow for replacement of the hazardous X-ray imaging commonly used in guiding catheters. The end result would be a more cost efficient, less dangerous procedure benefiting doctors, hospitals, and their patients.
Many different types of ultrasonic transducers such as thickness mode, polymer based, solid core and hollow core transducers are known in the art.
Thickness-mode ultrasonic transducers possess low signal loss, high sensitivity, and are inexpensive to produce. However, they are highly directional and as such, do not meet the omnidirectional requirement.
Polymer based piezoelectric materials can be easily fabricated into different shapes, including an omnidirectional geometry such as a sphere. However, polymer based piezoelectric materials possess high signal losses and low electromechanical coupling coefficients, which render polymer based transducers unsuitable for this use.
Solid core ceramic spherical transducers are omnidirectional. However, low sensitivity makes them less than suitable.
In contrast to the above transducers, hollow sphere ceramic electroactive transducers have the required omnidirectionality, low signal loss and high sensitivity required. In addition, they can be easily matched to electronic systems. Thus, hollow sphere ceramic electroactive transducers promise a natural solution to the problems associated with guiding catheters using ultrasound-imaging technology. However, given the current state of the art, it is extremely difficult to fabricate uniform hollow sphere ceramic electroactive transducers.
Presently, hollow sphere ceramic electroactive transducers can be produced by machining and grinding bulk ceramic into hemispheres. An electrode is then formed on the inner surface of the hemispheres, followed by bonding of two such hemispheres together. This approach is labor intensive, high cost, and low in productivity.
U.S. Pat. No. 4,917,857 to Jaeckel et al. is directed to a process for the production of metallic or ceramic hollow spheres to make a reticulate structure. In this process, foamed polymer cores are coated with a suspension containing metal or ceramic particles in a bed reactor. The polymer core is later pyrolyzed to obtain a metal or ceramic hollow sphere. A hollow sphere ceramic electroactive transducer can be prepared using hollow ceramic spheres obtained from this process by first opening a small hole in the sintered sphere, normally by polishing. Next, an inner electrode is deposited on the inner surface of the hollow sphere through the hole. An outer electrode is then deposited on the outer surface of the sphere. The ceramic is then poled under an electric field. However, this method, and the transducers produced by this method have low green density. In addition, the ceramic shells contain many pores due to the forming process that hinder subsequent sintering of the green ceramic body. The pores become defects in the surface of the sintered sphere, leading to mechanical fractures and decreased sensitivity when the spheres are processed into hollow sphere ceramic electroactive transducers.
Additionally, the hole created is mechanically weak. This weakness limits the hydrostatic pressure tolerance of the transducers produced in this fashion. When used in underwater applications, the depth capability of these transducers is drastically reduced.
Thus, as discussed above, the crude fabrication techniques present in the art do not allow for hollow ceramic electroactive transducers to be produced uniformly, in commercially significant quantities, at a cost low enough to make this technology appealing.